Activation of different Cl currents in Xenopus oocytes by Ca liberated from stores and by capacitative Ca influx.

Xenopus oocytes are an excellent model system for studying Ca signaling. The purpose of this study was to characterize in detail the Ca-activated Cl currents evoked by injection of inositol 1,4,5-trisphosphate (IP3) into Xenopus oocytes voltage-clamped with two microelectrodes. Injection of IP3 into Xenopus oocytes activates two different Ca-activated Cl currents. ICl-1 is stimulated rapidly (within 5 s after IP3 injection), exhibits time-dependent activation upon depolarization, a linear instantaneous IV relationship with a reversal potential near ECl, and a curvilinear activation curve with an approximate half-maximal activation voltage of > 200 mV. ICl-2D is stimulated slowly after IP3 injection (half-maximal stimulation occurs approximately 3 min after injection). ICl-2D has a strongly outwardly rectifying instantaneous IV relationship with a reversal potential near ECl and is activated by hyperpolarization with a half-maximal activation voltage of -105 mV. ICl-2D cannot be activated by Ca released from stores but is activated by Ca influx. In contrast, ICl-1 can be stimulated by Ca released from intracellular Ca stores. It can also be stimulated by Ca influx through store-operated channels if the Ca driving force is increased by a hyperpolarization immediately before the depolarization that gates ICl-1 channels. The description of two currents activated by influx and Ca release from stores provides new insights into and questions about the regulation of Ca in Xenopus oocytes.